Resonant systems present familiar design challenges. When trying to control a resonant system at a peak of a resonant mode, a feedback control circuit is commonly employed to produce an excitation signal in an attempt to keep the system resonating at a particular frequency and phase. Components within the feedback control circuit can have differing characteristics that increase the design challenges. For example, components may have characteristics that drift with time, age, and temperature. Further, the resonant frequency and phase shift of the system may vary with time, age and temperature.
Scanning laser projectors are an example of a resonant system. Scanning laser projectors typically employ a resonant mirror that oscillates in at least one dimension. Scanning laser projectors may employ parameter estimation circuits to estimate operating parameters such as oscillation amplitude, phase shift, or period. Estimated parameters may then be used to estimate the physical position of the resonating mirror over time. The image resolution of scanning laser projectors is limited by the accuracy with which the position of the resonating mirror can be estimated. As the position estimation accuracy increases, pixels can be placed more densely during mirror oscillation, thereby increasing image resolution.
Parameter estimation circuits typically include analog circuits such as filters, comparators, and peak detectors. Analog filters suffer from variations is manufacturing, phase drift over temperature, and phase drift over frequency. Analog comparators suffer from manufacturing variation, threshold drift over temperature, and variable delay based on signal amplitude. Threshold variation and threshold drift stacks up with the analog filter phase drift to further limit the ability of a projector to accurately place pixels. Analog peak-detector circuits suffer from the same disadvantages as analog filters and additionally suffer from aliasing, accuracy, and ripple problems. Variations in manufacturing result in complicated calibration steps which limit unit throughput and add manufacturing expense. Accordingly, the drawbacks of typical parameter estimation circuits function to limit the resolution of scanning laser projectors since they limit the accuracy with which pixels can be placed on a projection surface.